Catalytic oxidation of CO over Fe-, Sn-, and Ce-modified MnO2 catalysts

Manganese oxides are promising candidates to create new catalysts possessing activity in a relatively lowtemperature oxidation of formaldehyde and CO. Such materials are widely used in oxidative processes due to high oxygen mobility associated with the presence of redox pair of Mn(IV)/Mn(III), existing as oxygen vacancies [−Mn3+−□−Mn3+−]. A prospective way to further improve the catalyst performances is manganese oxide modification with transition metal cations or variable valence cations [1, 2]. The CO oxidation reaction is widely used as a test process for solving fundamental problems in the field of heterogeneous catalysis [3–4], being an effective tool for studying the dependence of catalytic activity on structural features of material, e.g. the size, morphology, states of modifying cations/clusters/nanoparticles [5–6]. The aim of this work is to study the synergetic effect between transition metal modifier (Fe3+, Sn4+, Ce4+) in CO oxidation process. All catalysts were characterized by BET, TEM (HRTEM), XRD, Raman and long-tested in CO oxidation process for 100 h to obtain TOS curves. Catalyst samples were prepared using a redox reaction between required amount of Mn(NO3)2 and KMnO4 according to procedure reported in [2]. According to XRD data, α-MnO2 crystalline phases were primarily formed for unmodified sample, Fe- and Sn modified samples. For Ce-modified catalyst β-MnO2 crystalline phases were prevailed. No reflexes of iron and cerium oxides were observed in the diffractograms of the Fe- and Ce-modified samples suggesting high dispersion of such phases. In Sn–MnO2 sample a dispersed SnO2 phase was identified. The lattice-resolved HRTEM showed the regular lattice arrangement of α-MnO2 and β-MnO2 for nanorod particles of unmodified sample. TEM images indicate nanorod, spindle-shaped and shapeless particles for Fe–MnO2, the nanorod particles and 3–7 nm nanoparticles agglomerated or located on the nanorod particles for Sn–MnO2 and nanorod particles and agglomerates of shapeless particles for Ce–MnO2. The TEM data indicates the incorporation of Ce in the manganese oxides rather the formation of CeO2 phase for Ce-modified sample, which comes in agreement with XRD data. Catalytic testing in CO oxidation process was carried out in quartz reactor according to procedure described in [7]. Catalysts were tested for 100 h. The temperature increased from 50 to 125 °C for Ce–MnO2 and to 145 °C for other catalysts. The molar ratio of O2/CO was 8, GHSV = 36000 h-1. It was shown that the introduction of the transition metal into the composition of MnO2 with the OMS-2 structure allowed increasing the activity of catalysts: MnO2 < Sn–MnO2 £ Fe–MnO2 < Ce–MnO2. All catalysts were stable in CO oxidation process during 100 h. Therefore, catalysts based on manganese oxides modified with Fe, Sn or Ce possess good catalytic properties in CO oxidation and are promising candidates to create effective oxide catalysts. Further research will focus on investigation of Fe- and Ce-containing OMS-2 catalysts modified by bimetallic (Ag–Pd) nanoparticles. This research project is supported by Russian Science Foundation (project 19-73-30026). References 1. Sultana, S.; Ye, Z. et al. Catal. Today 2018, 307, 20. 2. Dotsenko, S.S.; Verkhov, V.A. et al. Catal.Today 2019, in press. 3. Kropp, T.; Mavrikakis, M.; J. Catal. 2019, 377, 577. 4. Royer, S.; Duprez, D. Chem.Cat.Chem. 2011, 3, 24. 5. Nikolaev, S.A.; Golubina, E. et al.; Kinet. Catal. 2014, 55, 311. 6. Gordon, E. B.; Karabulin, A. V. High Energy Chem. 2016, 50, 305. 7. Grabchenko, M. V.; Mamontov, G. V. et al. Appl. Catal. B: Environmental 2020, 260, 118148.